The recently-signed Irish international was part of a backline that failed to get in the way of Kei Kamara free kick winner late in the game as Sporting Kansas City edged TFC 1-0 on Saturday.
Dead ball situations and giving up late goals have been Toronto's Achilles heel this season, and Kansas City made them pay once again in the 83rd minute.
"That's been the story of the season," O'Dea said after his debut. "We should have taken a point against a very good team. We've come away with nothing again and obviously it's disappointing."
Kamara blasted in a free kick that deflected off TFC midfielder Andrew Wiedeman and slipped through the fingers of 'keeper Milos Kocic.
"It's off my thigh. I was the fifth man of the wall," said Wiedeman. "It's tough. He hits it with such pace that a deflection like that throws everything off."
Kamara, who leads the team with nine goals, admitted it wasn't a highlight-reel marker, though enough to power Kansas City (14-7-4) to its third straight MLS victory.
"It's one of those set pieces that's not something that we crossed in and we finished it," he said. "It's one of those where ... I tried to hit it on frame, and it took a perfect reflection for it to go to the back of the net."
Sporting is atop the Eastern Conference and now unbeaten in five straight games. The team is rolling like a contender with the playoffs just over a month away.
"We're just feeling it," said Kamara. "We're top of the conference and we know if we keep playing our game thing are going to go our way. We want to finish the season being top of the league, and that's a goal."
Toronto, meanwhile, remains dead last in the Eastern Conference at 5-14-5 and is winless in its last four MLS games.
TFC gave up a corner kick goal — its seventh this season, most in the MLS — in a 2-2 draw against Portland on Wednesday, prompting head coach Paul Mariner to run set piece drills at practice Friday.
Despite SKC getting its winner on a restart, Mariner didn't want to put too much stock in the goal.
"It was a bit of nothing match to be perfectly frank," said Mariner. "It was a bit of a stalemate and then all of a sudden they get the goal. We pushed and pushed ... but they're a good team.
TFC came out strong in the second half as Eric Hassli immediately went to work fighting off defenders and battling for possession only to be shown a yellow card for his tenacious efforts. The physical French striker opened his TFC account with goal against Portland.
He almost got his second in the 54th with a flick off Ryan Johnson's cross, but the ball rolled a foot wide of the right post.
With many TFC players coming off mid-week international duty and legs looking tired, the pace slowed down with 30 minutes left to play.
Johnson woke up Kansas City 'keeper Jimmy Nielsen in the 70th when he headed a service pass from captain Torsten Frings just up slightly over the bar.
The game got off to a chippy start with two yellows in the first 15 minutes. Frings earned a yellow in the eighth by tackling a driving Graham Zusi from behind. The resulting free kick — an early test to Toronto's recent commitment to defend on set pieces — was blocked handily.
Four minutes later SKC defender Aurelien Collin was shown a yellow for a bad foul.
Kocic made a dazzling save in the 30th off a blazing right-footed shot from Kamara and then minutes later stifled a charging Teal Bunbury that had the BMO Field crowd roaring. The Hamilton-born forward was booed mercilessly, as locals continue to display their scorn for his decision to play for the U.S. national team.
Kocic finished the game with seven saves. SKC had 17 attempts on goal to Toronto's six.
O'Dea looked lively in his debut, despite having played an international friendly in Belgrade just three days earlier.
"I thought he equipped himself unbelievable well," said Mariner. "He's talking at the back, he's commanding the line. He's everything that I want. Absolutely everything."
Notes: Toronto defender Doniel Henry hurt his knee in the first half Saturday and will need an MRI. ... SKC's last meeting with the Reds was a 2-0 victory at home in Mariner's first game as manager after taking over from Aron Winter. ... Toronto is hitting the road for two games before returning to BMO for a Champions League tilt with Santos Laguna on Aug. 28. ... TFC's next game on Wednesday at Columbus is a makeup match after the original Aug. 10 meeting was rescheduled to allowed members of the Crew to attend the funeral of former teammate Kirk Urso. ... Attendance was 18,151.